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factsheet ISSUE 6
Hazardous explosive environments C L A S S I F I C AT I O N O F H A Z A R D O U S A R E A S ( Z O N I N G ) A N D S E L E C T I O N O F E Q U I P M E N T
ertain gases, vapours, mists and dusts can form explosive atmospheres with air. Hazardous area classiﬁcation is used to identify places where, because of the potential for an explosive atmosphere, special precautions over sources of ignition are needed to prevent ﬁres and explosions. Hazardous area classiﬁcation should be carried out as an integral part of the risk assessment to identify places (or areas) where controls over ignition sources are needed (hazardous places) and also those places where they are not (non hazardous places). Hazardous places are further classiﬁed in Zones which distinguish between places that have a high chance of an explosive atmosphere occurring and those places where an explosive atmosphere may only occur occasionally or in abnormal circumstances. The deﬁnitions of the Zones (which are included in DSEAR) also recognise that the chance of a ﬁre or explosion depends on the likelihood of an explosive atmosphere occurring at the same time as an ignition source becomes active.
DSEAR deﬁnes a place where an explosive atmosphere may occur in quantities that require special precautions to protect the health and safety of workers as hazardous. A place where an explosive atmosphere is not expected to occur in quantities that require such special precautions is deemed to be non-hazardous. For these purposes “special precautions” means precautions to control potential ignition sources within a hazardous area, particularly in relation to the construction, installation and use of equipment. The term “not expected to occur in such quantities” means that employers should consider the likelihood of releases of explosive atmospheres as well as the potential quantity of such releases when considering area classiﬁcation. So, if a release is extremely unlikely to occur and/or if the quantities released are small, it may not be necessary to classify the area as hazardous. For example, if a dangerous substance is being carried through a seamless pipe, and that pipe has been properly installed and maintained, it is extremely unlikely that the substance will be released. An explosive atmosphere would not be expected to occur from this source and the area surrounding the pipe would be non-hazardous. A spillage from a small bottle of solvent would release so little ﬂammable
material that no special precautions are needed other than the general control of ignition sources (for example, no smoking) and cleaning and disposing of the spillage. It would not be classiﬁed as a hazardous area. When considering whether hazardous area classiﬁcation is necessary for “small” quantities of dangerous substances the actual circumstances of use and any speciﬁc industry guidance should also be taken into account. Dangerous substances in small pre-packaged containers for sale, display, etc. in retail premises would not normally (“normal” is intended to ensure that atypical situations such as a poorly ventilated basement in a Shop etc. where aerosols or other ﬂammables are present remains subject to a hazardous area classiﬁcation study) require the area to be classiﬁed as hazardous. However, HSE would expect a hazardous area classiﬁcation to be carried out for prepackaged containers held in large quantities e.g. in warehouses. Procedures to clean up and dispose of any spillage/release and to control ignition sources in the event of such a release would be needed.
asses s i n g t h e r i s k Identifying hazardous or non-hazardous areas should be carried out in a systematic way. Risk assessment should be used to determine if hazardous areas exist and to then assign zones to those areas. The assessment should consider such matters as: (a) the hazardous properties of the dangerous substances involved; (b) identiﬁcation of the possible sources of release; (c) the amount of dangerous substances involved; (d) the work processes, and their interactions, including any cleaning, repair or maintenance activities that will be carried out; (e) the temperatures and pressures at which the dangerous substances will be handled; (f) the containment system and controls provided to prevent liquids, gases, vapours or dusts escaping into the general atmosphere of the workplace; (g) any explosive atmosphere formed within an enclosed plant or storage vessel; and, (h) any measures provided to ensure that any explosive atmosphere does not persist for an extended time, e.g. ventilation. Taken together these factors are the starting point for hazardous area classiﬁcation, and should allow for the identiﬁcation of any zoned areas. Relationship between ﬁres and explosions In many cases where an explosive atmosphere can form, any ignition will cause a ﬁre rather than an explosion. Both ﬁre and explosion cause danger to workers, and in many cases the precautions required to prevent an ignition are the same. The overall package of precautions required will depend on the possible consequences of a ﬁre or explosion. Many factors inﬂuence the risks from a ﬁre involving dangerous substances. In particular, employers should consider whether a ﬁre could lead to an explosion, how fast a ﬁre might grow, what other materials might be rapidly involved, any dangers from smoke and toxic gases given off, and whether those in the vicinity would be able to escape.
Classifying hazardous areas into zones Once an area has been identiﬁed as hazardous it should be classiﬁed into zones based on the frequency and persistence of the potentially explosive atmosphere. This then determines the controls needed on potential sources of ignition that may be present or occur in that area. These controls apply particularly to the selection of ﬁxed equipment that can create an ignition risk; but the same principles may be extended to control the use of mobile equipment and other sources of ignition that may be introduced into the area (for example, portable appliances, mobile equipment, matches and lighters) and the risks from electrostatic discharges. An international standard, BS EN 60079/10, explains the basic principles of area classiﬁcation for gases, vapours and mists and IEC 61241/10 for dusts. These standards form a suitable basis for assessing the extent and type of zone, and can be used as a guide to complying with the requirements in DSEAR. However, they cannot give the extent and type of zone in any particular case, as site-speciﬁc factors should always be taken into account. Industry speciﬁc codes have also been published by various organisations and, provided they are applied appropriately, they are valuable in encouraging a consistent interpretation of the requirements. Area classiﬁcation studies usually take the form of drawings identifying the hazardous areas and zones. Additional text gives information about the dangerous substances that will be present, the work activities that have been considered, and other assumptions made by the study. Whenever such drawings and documents have been produced, they should be included in the risk assessment record required by DSEAR. These documents should be considered whenever new equipment is to be introduced into a zoned area. Maintenance During a period when maintenance is being carried out the normal area classiﬁcation drawings may not be applicable. If dangerous substances have been removed, it may be possible to treat areas normally classiﬁed as hazardous as non-hazardous. Alternatively, if the maintenance creates the larger than normal risk of a release of a dangerous substance, larger areas may need to be treated as hazardous. It is not normally necessary to create new area classiﬁcation drawings for the duration of the maintenance work. Equipment in hazardous areas Special precautions need to be taken in hazardous areas to prevent equipment from being a source of ignition. In situations where an explosive atmosphere has a high likelihood of occurring, reliance is placed on using equipment with a low probability of creating a source of ignition. Where the likelihood of an explosive atmosphere occurring is reduced, equipment constructed to a less rigorous standard may be used. Equipment is categorised (1, 2 or 3) depending on the level of zone where it is intended to be used. A number of ways of constructing equipment to prevent ignition risks have been published as harmonised European Standards, and in some cases, additional requirements are set out in the standards relating to installation and use. Other areas to consider are the marking and selection of equipment for use in hazardous areas. For more information please visit the HSE website at www.hse.gov.uk.
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